1. Field of the Invention
The present invention relates to a substrate installed in an ink jet print head and to an ink jet print head that ejects ink to perform printing.
2. Description of the Related Art
Ink jet printers of recent years are required to form high-resolution images and at the same time print them at higher speed. To meet both requirements—higher resolution of images and faster print speed—one method is available that performs printing at low resolution using large ink droplets in a fast mode and, in a fine mode, performs printing at high-resolution using small ink droplets.
Selecting an optimal print mode on the part of the user to produce a desired image output is very useful. An ink jet print head that realizes this technique is disclosed in Japanese Patent Laid-Open No. 2004-122757.
An ink jet print head disclosed in Japanese Patent Laid-Open No. 2004-122757 (corresponding to U.S. Pat. Nos. 6,966,629 and 7,144,093) has one heater for each nozzle, with first and second heaters arranged alternately in one column extending in a predetermined direction. The first and second heaters can be selectively driven by a select signal, making it possible to produce an image with a wide range of gradation. This ink jet print head drives the first and second heaters of different electric resistances with one and the same power supply. This allows the first and second heaters to use common power supply wires, simplifying a circuit configuration, reducing cost and the size of the head.
FIG. 16 is a plan view of a conventional wiring configuration in an ink jet print head substrate having the first and second heaters arranged alternately in one column, extending in a predetermined direction. In FIG. 16, heaters 103 installed in a base layer 101 each have a first, a second heater, and electrode wires to supply electricity to them. One of wiring portions in each heater 103 is connected to one of power supply side wiring portions 104a, 104b, 104c, 104d. The other wiring portion of each heater 103 is connected to a drive element 108 formed of a switching device such as a transistor. The drive element 108 is further connected to common electrodes 105a, 105b, 105c, 105d on the grounding side.
Upon receiving a signal from a drive circuit described later, the drive element 108 selectively drives the heaters 103 according to print data to eject ink from the corresponding ejection orifices. The power supply side wiring portions 104a, 104b, 104c, 104d and electrodes 105a, 105b, 105c, 105d are connected to electrode pads 107, through which they are connected to a power supply and a grounding circuit, respectively. The grounding side electrodes 105a, 105b, 105c, 105d and the corresponding power supply side wiring portions 104a, 104b, 104c, 104d are constructed so that respective wiring resistances between the grounding side electrodes 105a, 105b, 105c, 105d and the corresponding power supply side wiring portions 104a, 104b, 104c, 104d are equal to each other. Thus, in this ink jet print head, which has the first heaters and the second heaters 103 of different electric resistances arranged on both sides of an ink supply port 102 as shown in FIG. 16, whichever of the first and second heater is selected, a voltage drop in the wiring portion will not vary either on the power supply side or on the grounding side. This in turn obviates the need to increase the wire width in coping with a voltage drop that would result when the first and second heaters are driven simultaneously, thus allowing for a reduction in the size of the print head.
FIG. 17 is a circuit block diagram of an example conventional ink jet head substrate.
The circuit shown in FIG. 17 has input terminals, such as a heater drive signal input terminal 401, a clock (CLK) input terminal 402, a data input terminal 403, a selection circuit 404, and a latch signal input terminal 405. It also has a heater voltage input terminal 406, a drive circuit 407, a selection data transfer circuit 408, a selection data hold circuit 409, a decoder 410, a data transfer circuit 411, a holding circuit 412, an AND circuit 413, and heaters A, B.
The heaters A, B are the first heater 103 and the second heater 103 shown in FIG. 16. There are m heater groups, each made up of 2n kinds of heaters (in this case, two kinds)—first and second heaters A, B. In each group, the drive circuit 407 and the AND circuit 413 are provided for each of the heaters A, B. The drive circuit 407 drives the heaters A, B according to an output of the AND circuit 413.
In the above circuit, a heater group and a heater kind are chosen according to data entered into the data input terminal 403 and, based on the input data, the first heater A and second heater B are driven. That is, the selected data transfer circuit 408 outputs heater group selection data to the decoder 410 through the selected data hold circuit 409, and also outputs heater kind selection data to the selection circuit 404. Further, the selected data transfer circuit 408 outputs data for printing an image to the data transfer circuit 411. The holding circuit 412 and the data transfer circuit 411 are commonly used by both heaters A and B. Switching between the first heater A and the second heater B is determined by the data entered into the selected data transfer circuit 408 through the data input terminal 403, and a selection of the first or second heater is made by the selection circuit 404.
In FIG. 17, a heater drive power is supplied to the heater voltage input terminal 406. The heater drive power is connected to the ends of the first and second heaters A, B of all groups S(1)-S(m) through a common wire. Input to the data transfer circuit 411 through the selected data transfer circuit 408 is serial image data from the data input terminal 403 that corresponds to each of the groups S(1), S(2), . . . , S(m). Also supplied to the data transfer circuit 411 through the selected data transfer circuit 408 is a clock input signal from the clock input terminal 402 to drive the data transfer circuit. The input image data is then output to the holding circuit 412 as a parallel signal.
The holding circuit 412 is supplied a latch signal through the latch signal input terminal 405. The holding circuit 412 temporarily holds the image data entered from the data transfer circuit 411 before outputting it to the AND circuit 413 for the corresponding group S(1), S(2), . . . , S(m). A drive pulse signal input to the heater drive signal input terminal 401 is supplied to the first heaters A and second heaters B of the groups S(1)-S(m).
As described above, the data input to the selected data transfer circuit 408 from the data input terminal 403 includes, in addition to image data, a signal representing a selection group and kind of heater to be driven. This selection signal is 5-bit long and output to the selected data hold circuit 409. The selected data hold circuit 409 outputs 4 bits of the 5-bit signal received to the decoder 410 and a 1-bit signal representing the kind of heater to be driven to the selection circuit 404.
The decoder 410 has its output terminals divided and connected to each of AND circuits of the groups S(1)-S(m) so that it can determine the group to be connected according to the 4-bit signal received. The selection circuit 404 selects the kind of heater (heater A or B in this case) making up each group. That is, the selection circuit 404 outputs the received 1-bit signal as is to the AND circuit for the first heater A and at the same time inverts the 1-bit signal by an inverter before supplying it to the AND circuit 413 for the second heater B. This prevents the first heater A and the second heater B from being selected simultaneously to ensure that only one of them is selected.
Therefore, the wires for the power supply side wiring portions 104a, 104b, 104c, 104d and the grounding side electrodes 105a, 105b, 105c, 105d, connected to the associated groups, carry only enough current to drive either the first heater A or second heater B at the same time. The voltage drops caused by the wiring resistances of the individual electrodes are of the same value, which means that power losses due to the wiring resistances are equal for all groups, thus preventing adverse effects on ink ejection characteristics.
It has also been proposed that large ink droplets and small ink droplets are used simultaneously to print an image with high gradation at high speed.
With the print head shown in FIG. 16 and FIG. 17, however, since there is a limitation that the heaters that can be driven simultaneously in each group are only either first or second heater, there remains room for improvement in forming printed images with high gradation.
In forming an image with high gradation, the first and second heaters that eject different volumes of ink may be driven simultaneously to eject large ink droplets and small ink droplets at the same time. That is, by selectively landing large ink droplets and small ink droplets at desired positions, improvements can be made of image quality variations caused by ink droplet size variations, which are caused by head fabrication variations, landing position variations, and mechanical precision variations in a printing apparatus body.
If there is a limitation that only one of the first and second heaters can be driven at a time, a degree of freedom in image design is degraded. That is, under the limitation that the ink droplets that can be ejected simultaneously are only large ink droplets or only small ink droplets, it is not possible to adopt an image design method of improving an apparent image quality by mixing different sizes of ink droplets. So, if such a limitation exists, there is a possibility of not being able to make improvements in image quality above a predetermined level.
Hence, to be able to produce an image with high gradation, a method may be employed that involves entering a selection signal, that can individually distinguish between the first heaters A and second heaters B, into the AND circuit 413. With this method, it is possible to drive the first heaters A and second heaters B individually or simultaneously. In addition to the selection signal, a group selection signal corresponding to each of the first heaters A and second heaters B may be provided. This enables the first heaters A and second heaters B in each group to be driven either individually or simultaneously.
However, in the circuit shown in FIG. 17, the power supply side wiring portions and the grounding side electrodes form common wires for the first and second heaters in the same group. Thus, if the first and second heaters are made individually selectable and if both of them with different electric resistances are driven simultaneously, as described above, a voltage drop occurs in each of the wiring portions connected to the electrodes according to a combined value of currents flowing through the heaters. Thus, a voltage drop occurring in each wire when the first heaters A and second heaters B are driven simultaneously differs from (i.e., is greater than) the one that occurs when the heaters are driven individually or separately.
If arrangements are made so that the first heaters A and second heaters B can be driven individually in each group, a voltage drop caused by a wire resistance varies from one group to another. This means that a resulting power loss differs among different groups, with an energy applied to individual heaters also varying among the groups. Then, the difference in energy applied to individual heaters will result in variations in ink droplet ejection characteristics, rendering ink ejections unstable, which in turn causes a phenomenon of ink droplet landing variations. This is against the object of this method of forming an image with high gradation by using large ink droplets and small ink droplets at the same time.
Under this circumstance, a method is being studied which arranges separately for each heater those wires of the power supply side wiring portions and the grounding side electrodes in each group of the first and second heaters and which sets the wire resistances individually to make them equal.
However, in a print head substrate having first and second heaters alternated or three or more kinds of heaters arranged repetitively, if one tries to connect individual heaters two-dimensionally to the corresponding power supply side wiring portions and grounding side electrodes, the area required for wiring becomes prohibitively large. That is, if the first and second heaters are arbitrarily driven in each group, although voltage drops caused by wiring resistances can be made uniform among different groups, an increase in substrate size and a substantial rise in substrate cost are inevitable.